1. Field of the Invention:
The present invention relates to a hydraulic device for use in a working machine such as a hydraulic shovel or the like, and more particularly to a hydraulic device for use in a working machine capable of carrying out building, construction, and civil engineering work with actuators such as hydraulic cylinders and a hydraulic motor which can be operated by the operator with control levers.
2. Description of the Related Art:
Hydraulic shovels have actuators such as hydraulic cylinders for actuating an arm, a boom, and a bucket, and a hydraulic motor for turning some members. These actuators are operated by oil under pressure which is discharged by a hydraulic pump that can be driven by an engine mounted on a motor vehicle. The actuators can operate in different directions which are selected by directional control valves that are controlled by control levers connected to pipes extending from the hydraulic pump to the actuators.
Usually, each of the directional control valves is supplied with a pilot pressure proportional to the amount of controlling movement of the corresponding control lever. The supplied pilot pressure displaces a spool of the directional control valve to equalize the opening area (opening) of the directional control valve to a desired opening area which is in proportion to the amount of controlling movement of the control lever. One generally known type of hydraulic device for use in such a working machine has a pressure compensator for controlling the difference between the pressure at the outlet of the directional control valve, i.e., the load pressure on the actuator, and the pressure at the inlet of the directional control valve so as to be equal to a preset differential pressure, so that the flow rate of oil supplied to the actuator is proportional to the opening area of the directional control valve, i.e., the amount of controlling movement of the control lever, irrespective of the magnitude of the load pressure on the actuator.
The operator of the working machine should preferably have a choice of available flow rate gains for oil supplied to the actuator with respect to the amount of controlling movement of the control lever, i.e., the operator should be capable of selecting a desired one of rates of change of the flow rate with respect to the amount of controlling movement of the control lever. The operator should also be able to quickly recognize the load condition of the actuator through the operation of the control lever.
The conventional hydraulic device with a pressure compensator allows the operator to adjust a setting for the differential pressure between the inlet and outlet of the directional control valve for thereby adjusting the flow rate gain. Since the opening of the directional control valve is proportional to the amount of controlling movement of the control lever, however, the amount of controlling movement of the control lever which starts to supply oil to the actuator to operate the actuator is constant regardless of the magnitude of the load on the actuator. Consequently, the operator cannot recognize the load condition of the actuator through the controlling of the control lever.
It is possible to adjust the flow rate gain when the differential pressure between the inlet and outlet of the directional control valve is automatically set depending on the load pressure on the actuator. The operator can now recognize the load condition of the actuator because the operational speed of the actuator varies depending on the load thereof even if the amount of controlling movement of the control lever remains the same. However, inasmuch as the flow rate gain is determined in advance depending on the load imposed on the actuator, it is not possible to operate the actuator in a manner to meet the operator's preference or various working patterns.
Some hydraulic devices for use with working machines are of the so-called bleed-off control type with no pressure compensator. In this type, the amount of controlling movement of the control lever which starts to operate the actuator varies depending on the load imposed on the actuator. Since the flow rate gain simultaneously .varies depending on the load on the actuator, the flow rate gain cannot be adjusted to meet the operator's preference or various working patterns.